A leading House Democrat revealed Wednesday he was approached by a whistleblower who said a businessman received a text from Lt. Gen. Michael Flynn on Jan. 20, 2017 at 12:11 pm – as the newly sworn President Donald Trump was giving his inaugural address – indicating that a lucrative nuclear energy plan they had been developing with Russian partners was “Good to go.”

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The whistleblower alleged that the businessman boasted about the contents of the text from Flynn, calling it “the best day of my life,” according to a detailed account of the alleged incident laid out in a letter from Rep. Elijah Cummings, the ranking Democrat on the House Oversight committee, to Chairman Trey Gowdy, a Republican, and shared with ABC News.

The whistleblower said he noted the time stamp, but did not read the text.

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Photo handed out by House Democrats highlighting Michael Flynn at President Trump's inauguration, Jan. 20, 2017.

“Our committee has credible evidence that President Trump’s National Security Advisor sought to manipulate the course of international nuclear policy for the financial gain of his former business partners,” wrote Cummings. “These grave allegations compel a full, credible, and bipartisan congressional investigation.”

Flynn pleaded guilty last week to lying to the FBI, and pledged to cooperate with Special CounselRobert Mueller in his investigation into Russian election interference. That agreement made no mention of Flynn’s consulting work for companies pursuing a controversial nuclear power project in the Middle East that involved Russian participation.

Reached Wednesday, Flynn’s attorney declined to comment.

The new account offers a window into the range of allegations being shared with the FBI as agents prepared to negotiate a cooperation agreement with Flynn. Flynn’s consulting firm, the Flynn Intel Group, had numerous clients exploring business ventures that would require some involvement from federal government agencies.

One of them, a proposed venture to help Middle Eastern nations build nuclear power plants with assistance from Russia and the U.S., garnered significant press attention. The Wall Street Journalreported in September that despite records showing Flynn’s work on the project ended in December 2016, Flynn continued promoting the plan to colleagues inside the White House.

The whistleblower told congressional investigators he was with Alex Copson, a former business associate of Flynn’s, watching the Trump inauguration ceremony at an event in Washington, according to the letter. The whistleblower alleged Copson showed off the text to those around him, calling it “the best day of my life.”

Copson allegedly told those around him that “Mike [Flynn] has been putting everything in place for us. This is going to make for a lot of very wealthy people,” Cummings’s letter quotes him as saying.

The whistleblower told congressional investigators that Copson boasted that Flynn was “making sure that sanctions would be ‘ripped up’ as one of his first orders of business and that this would allow money to start flowing into the project,” an apparent reference to financial sanctions imposed on Russia by the Obama administration, Cummings’s letter states.

Copson, who worked at the firm ACU Strategic Partners, refuted the whistleblower’s assertion in a statement to ABC News from his lawyer, Donald Gross.

“For the record, no member of ACU received any communication in any form from General Flynn during the presidential campaign, the presidential transition, the Inauguration, the period following the Inauguration when General Flynn served as national security adviser or subsequent to General Flynn's resignation,” the statement said.

House investigators turned over the information they had gathered to the Mueller team several weeks ago. Prosecutors “asked my office to hold on the public release of this information until they completed certain investigative steps,” Cummings wrote. “They have now informed us that they have done so.”

Cummings wrote the letter Wednesday to Gowdy to seek support for a subpoena to the White House for records. The investigators are searching for further evidence that Flynn lobbied from his government job as National Security Advisor for associates who had retained his consulting firm before the election. Once Flynn started at the White House, federal regulations prohibited him from working on any matter he served as a “consultant” on during the previous year, according to the Office of Government Ethics.

Gowdy has pressed back previously on the demands for records, saying he does not want to impede the Special Counsel investigation.

Gowdy responded to Cummings’s letter late Wednesday, saying he did not believe the matter was in the jurisdiction of the House Oversight committee, but rather should be handled by the House Intelligence committee or the Special Counsel.

“What you allege here is a crime, squarely within the jurisdiction of the Special Counsel,” Gowdy wrote. “If you have evidence of a crime, you should provide it to the Special Counsel immediately.”

Cummings has argued that the Flynn plea deal should alleviate that concern. Cummings said he believes Republican leaders of the Oversight Committee should approve a subpoena so they can “pursue these allegations against General Flynn in a responsible way.”

The whistleblower is not named in the five-page letter that Cummings shared with ABC News on Wednesday, and is unlikely to identify him or herself, fearing retaliation. “I have spoken to this whistleblower, and I find this person to be authentic, credible, and reliable,” Cummings wrote.

Gates (not a Trump appointee) reported to Trump that Flynn had been compromised by the Russians and Trump did nothing. It was only after Flynn's lying became public that Trump fired him "because Flynn lied to the vice president." In that way, Trump had plausible deniability. Pence knew but he didn't. If you believe that, you'll believe anything.

What I can't figure out regarding Flynn is when he was in the military, who did he think was our enemy? Most likely he wouldnt remember the Cuban Missile Crisis but he lived in an era that had and to the best of my knowledge, viewed Russia as more of an enemy than any kind of ally. That being said, now with the allegations against him, I feel treason, Betraying your own country, is more in line with what Flynn should be charged with. What military secrets did he or could he have passed on to the Russians?